In His instructions to Śrīla Rūpa Gosvāmī (Cc. Madhya 19.138-48), Lord Caitanya graphically describes the rarity of gaining the association of a pure devotee. The Lord tells Rūpa Gosvāmī that there are unlimited living entities among 8,400,000 species, and all these living entities are wandering from body to body, planet to planet, within this universe. The few living entities in human bodies may be divided into the uncultured and the cultured—those who are ignorant of the Vedic principles and those who know them. Among those who know the Vedic principles, roughly half simply give lip service to these principles while committing all kinds of sins in violation of these principles. Out of those who actually follow the Vedic principles, most seek material rewards like wealth, good birth, or elevation to heaven. Among millions of pious followers of the Vedic injunctions, one may be actually wise (a jñānī). Out of many millions of such jñānīs, Lord Caitanya says, one may actually become liberated from birth and death, and out of many millions of such liberated persons, a devotee of the Lord is very difficult to find.

Lord Kṛṣṇa makes the same point:

manuṣyāṇāṁ sahasreṣu kaścid yatati siddhaye
yatatām api siddhānāṁ kaścin māṁ vetti tattvataḥ

"Out of many thousands of men, one may endeavor for perfection, and of those who have achieved perfection, hardly one knows Me in truth" (Bg. 7.3). This indicates that even one who has attained Brahman realization falls far short of knowledge of the Supreme Personality of Godhead. We should not be misled, therefore, about who is a "great soul" or think that any "swami" or "guru" will be able to deliver us from material entanglement. As the Śrīmad-Bhāgavatam says (6.14.5),

muktānām api siddhānāṁ nārāyaṇa-parāyaṇaḥ
su-durlabhaḥ praśantātmā kotiṣv api mahā-mune

"O great sage, out of many millions of materially liberated people who are free from ignorance, and out of many millions of siddhas who have nearly attained perfection, there is hardly one pure devotee of Nārāyaṇa. Only such a devotee is actually completely satisfied and peaceful."

Even when mahātmās do appear in human society, they are often not appreciated or understood. Śrīla Prabhupāda writes,

Sometimes devotees are personally attacked with violence. Lord Jesus Christ was crucified, Haridāsa Ṭhākura was caned in twenty-two marketplaces, and Lord Caitanya's principal assistant, Nityānanda, was violently attacked by Jagāi and Mādhāi.... Although a sādhu is not inimical toward anyone, the world is so ungrateful that even a sādhu has many enemies. [SB 3.25.21, purport]

But if one gets the association of a mahātmā and is receptive to his blessings, one will infallibly be benefited. Nārada is an excellent example of a mahātmā who transformed the lives of many. He once turned a hunter into a pure Vaiṣṇava. The hunter was so cruel that he used to half kill animals because he enjoyed their pain. But as soon as he met Nārada and began to hear from him, the hunter became afraid of his sins. Nārada assured him, "If you follow my instructions, you can be liberated." Nārada then instructed the hunter to worship Lord Kṛṣṇa by chanting the Hare Kṛṣṇa mantra. A mahātmā never says, "Surrender to me," but he advises everyone to surrender to Lord Kṛṣṇa, the Supreme Personality of Godhead. This is his infallibility.

The power of the Vaiṣṇavas is expressed in a Bengali song beginning gaurāṅgera bhakta-gaṇi jani jani śakti dhare: "The devotees of Lord Caitanya are very powerful, and every one of them can deliver the whole world." But the disciple has to do his part also. On receiving the grace of a Vaiṣṇava, one must agree to give up his sinful activities. Then the spiritual master can take care of him and elevate him to spiritual emancipation. Devotees who may not be on the level of a paramahaṁsa like Nārada Muni, but who strictly follow in his disciplic succession, can also deliver infallible knowledge. Śrīla Prabhupāda writes:

The spiritual master, being in the disciplic succession stemming from Nārada Muni, is in the same category with Nārada Muni. A person can be relieved of his sinful activity if he surrenders to the lotus feet of a person who actually represents Nārada Muni. [Cc. Madhya 24.258, purport]

Another proof of the power of the mahātmā is his ability to convert nondevotees into saintly persons. Śrīla Bhaktivinoda Ṭhākura stated that a Vaiṣṇava can be tested by seeing how good a "touchstone" he is—by seeing how many Vaiṣṇavas he has made during his life. Lord Caitanya desired that as many persons as possible should repeat the message of Kṛṣṇa and convince others to take up Kṛṣṇa consciousness, following in the footsteps of Nārada Muni and other great ācāryas.

In conclusion, the association of a mahātmā is very rare, and yet it is available to a sincere seeker. Upon contacting a great soul, one should realize one's good fortune, and with a joyful but serious attitude one should surrender unto his lotus feet. How one should regard a mahātmā upon meeting him is exemplified in this quote from the Hari-bhakti-sudhodaya (13.2), spoken by Lord Caitanya to Sanātana Gosvāmī:

My dear Vaiṣṇava, seeing a person like you is the perfection of one's eyesight, touching your lotus feet is the perfection of the sense of touch, and glorifying your good qualities is the tongue's real activity, for in the material world it is very difficult to find a pure devotee of the Lord.